Static dissipative nonwoven textile material

ABSTRACT

A nonwoven textile material useful in dissipating static electric charges containing an electrically conductive polytetrafluoroethylene fiber. An amount of a synthetic fiber may be intermixed with the electrically conductive polytetrafluoroethylene fiber to produce the static dissipative textile material. The nonwoven static dissipative textile material may combine with a porous polymeric membrane to form nonwoven fabric laminate. 
     The nonwoven textile material is useful as a filtration media and may be fabricated into a filter bag.

This application is a division of Ser. No. 07/952,939, filed Sep. 29,1992, now U.S. Pat. No. 5,229,200 which in turn is a division ofapplication Ser. No. 07/809,891 filed Dec. 18, 1991, now U.S. Pat. No.5,213,882.

FIELD OF THE INVENTION

This invention relates to a nonwoven textile material containingelectrically conductive polytetrafluoroethylene fibers. The nonwoventextile material may be in the form of a felt and is useful indissipating static electric charges. The nonwoven textile material maybe combined with a porous polymeric membrane to form a laminate.

BACKGROUND OF THE INVENTION

Control of static electricity can be of great importance in manyindustrial settings where an uncontrolled electrostatic discharge (ESD)or spark can result in serious damage. For example, static dischargescan bring about the destruction of integrated circuits during somestages of their manufacture. In explosive environments such as in grainelevators or in flammable environments such as an oil drilling rig, inrefineries and in solvent-based processes, a static discharge can beextremely dangerous and must be prevented in order to safeguard life andproperty.

Organic polymeric textile materials used in these settings can be thesource of static discharges due to the normally insulative nature of thematerials and have a high value of specific resistance, typically on theorder of 10¹⁶ ohm cm, unless the materials are altered to prevent buildup of electrical charges on their surfaces by permitting charges foundon their surfaces to drain in a controlled manner. To control staticelectrical charges found in textile materials, electrical conductivityof organic polymeric textile material may be increased throughapplication of antistatic finishes to the textile material or throughintroduction of fibers, which have a degree of electrical conductivity,into the textile material. Other means for controlling static electriccharges include such external devices as grounding straps or wires tocarry electrical charges found on the textile material to ground.

Antistatic finishes are commonly applied to organic polymeric textilematerials either when the organic polymeric textile material is in fiberform or in fabric form. These finishes typically increase ionicconductivity of the surface on which they are applied thereby promotingstatic dissipation. However, these finishes are typically not as durableas the polymeric textile materials on which they are applied. Use andcleansing of the organic polymeric textile material can remove thesefinishes from the fabric surface thereby resulting in a loss of theorganic polymeric textile material's ability to dissipate staticelectric charges found on their surfaces.

Coatings of metals or of conductive carbon may be added to the outsidesurface of fibers used in producing organic polymeric textile material.However, if the coating used is not as flexible as the fiber on which itis placed, flexing of the fiber while in use may cause cracking in thecoating and therefor interruptions in the conductive pathway formed bythe coating with a resulting loss of conductivity in the organicpolymeric textile material.

Introduction of fibers produced of materials that have a degree ofelectrical conductivity into the organic polymeric textile materialallows a more durable method of altering the surface conductivity of thematerial. Certain types of carbon and metal fibers are inherentlyconductive, and incorporation of these fibers even at a small percentageof the total fiber content of the textile material into a organicpolymer textile material can increase the conductivity of the textilematerial to the extent that a previously static-prone material can berendered static dissipative.

However, use of these fibers presents additional problems. Carbon fibersare relatively brittle when compared to the majority of fibers presentlyin use and exhibit the tendency to break when flexed. If the textilematerial is used in a high efficiency particle filter in a cleanenvironment, breakage of carbon fibers results in a reduction in thestatic dissipative capabilities of the organic polymeric textilematerial as well as providing a source of contamination to theenvironment which is known as media migration. This contamination can beespecially burdensome if the organic polymeric textile material is beingused in a clean room for the production of integrated circuits or forthe assembly of highly critical optical parts.

Use of metal fibers, usually stainless steel, also presents problems. Ifthe textile material is incorporated into a filter structure, metalfibers may be lost through media migration from the textile material andcontaminate the collected product and/or the filtered stream. This wouldpose a significant downstream problem to such industries as foodprocessors, pharmaceutical manufactures and chemical producers.

Metal fibers also present a problem for those who process textilematerials. Processors typically utilize metal detectors in theirequipment to detect the presence of stray pieces of metal present,commonly known in the art as "tramp metal". These stray pieces of metalif undetected can result in a flawed textile material as well as posinga hazard to equipment and personnel that would subsequently come incontact with the textile material. Use of metal fibers precludes the useof these detectors when producing an organic polymeric textile materialcontaining these metal fibers.

Organic polymeric fibers commonly used to produce organic polymerictextile materials, such as polyamides and polyesters, have hadconductive material incorporated into their structures to impart adegree of conductivity to these materials. The conductive material maybe found in a stripe of conductive material on the outside of the fiber,a conductive core of material found in the fiber or an amount ofconductive material dispersed throughout the fiber's structure. However,these materials have a relatively limited usable range with respect tothe temperatures and chemical exposure these materials can withstandwithout degrading. In certain areas where organic polymeric textilematerials are used for example as filter media, the need for chemicaland temperature resistance is great, and the need for a flexible,electrically stable, static dissipative textile material is critical.

Polytetrafluoroethylene (PTFE) is an organic polymeric material that ishighly resistant to many corrosive chemicals and remains stable over awide range of temperatures. PTFE, and especially a particular highstrength form of PTFE, expanded porous polytetrafluoroethylene, (ePTFE),has demonstrated utility as an organic polymeric textile material foruse in demanding environments. However, ePTFE is also an excellentinsulator and is commonly used as an insulating layer in electroniccable constructions.

Static dissipative textile materials that are highly resistant tocorrosive attack while remaining electrically stable and flexible over awide range of temperatures is the object of the present invention.

BRIEF DESCRIPTION OF THE INVENTION

A static dissipative nonwoven textile material comprising anelectrically conductive polytetrafluoroethylene fiber wherein saidstatic dissipative nonwoven textile material has a static dissipationtime of 0.5 seconds or less is disclosed. The static dissipative textilematerial is in the form of a nonwoven fabric, such as a felt. An amountof a synthetic fiber may be intermixed with the electrically conductivepolytetrafluoroethylene fiber.

The static dissipative nonwoven textile material may be combined with aporous polymeric membrane to produce a nonwoven textile laminate. Thenonwoven textile laminate is static dissipative when tested. The staticdissipative nonwoven textile material and the nonwoven textile laminatehave utility as filter media and may be fabricated into filter bags orfilter cloths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the inventive static dissipativenonwoven textile material in the form of a filter bag.

FIG. 2 is a perspective view of the inventive nonwoven textile laminatein the form of a filter bag.

DETAILED DESCRIPTION OF THE INVENTION

A static dissipative nonwoven textile material of the instant inventioncomprising an electrically conductive PTFE fiber, preferably anelectrically conductive ePTFE fiber. The ePTFE fiber, which normally isnonconductive, is rendered conductive through the incorporation of aparticular filler within the fiber. The term "fiber" is defined hereinas to include any slender filament and thus includes continuousmonofilament, tow, staple and flock. A conductive ePTFE fiber isproduced from an ePTFE matrix in film form in which an amount of aconductive particulate is contained.

The ePTFE matrix in film form is produced in the following manner:

A fine powder PTFE resin is combined with a conductive particulatethrough one of two methods. The conductive particulate having utility inthe electrically conductive ePTFE fiber may be selected from a groupconsisting of metals, metal oxides, graphites or carbon blacks. By"particulate" is meant individual particles of any aspect ratio and thusincludes flock, flakes and powders.

It is preferable to combine fine powder PTFE resin with the mineralspirit prior to the addition of the conductive particulate to theblender in order to obtain a consistent mixture of the fine powder PTFEresin and the conductive particulate. In another method, an aqueousdispersion PTFE resin is obtained. Into the aqueous dispersion, aconductive particulate is added. The mixture is co-coagulated by rapidshearing of the aqueous dispersion, or through destabilization of theaqueous dispersion with salt, acid, polyethylene imine or the like. Acoagulum of fine powder PTFE resin and conductive particulate issubsequently formed and dried into cakes. When dry, the cakes arecarefully crumbled and lubricated with a mineral spirit and blendedforming a compound.

The compound produced by either of the previously described methods iscompressed into a billet and subsequently extruded through a die by aram-type extruder forming a coherent extrudate. The mineral spiritfunctions as an extrusion lubricant for the compound.

The coherent extrudate is compressed between a pair of calender rollersto reduce its thickness. Subsequently, the mineral spirit is removedfrom the calendered coherent extrudate by passing the coherent extrudateover a series of heated rollers. The heated rollers are heated to atemperature at or above the boiling point of the mineral spirit presentin the coherent extrudate thereby volatilizing the mineral spiritleaving a dry coherent calendered extrudate.

The dry coherent calendered extrudate is stretched using the method ofexpanding PTFE taught in U.S. Pat. No. 3,543,566 to Gore incorporatedherein by reference. The dry coherent calendered extrudate is initiallyrapidly stretched uniaxially in a longitudinal direction 1.2× to 5000×,preferably 2× to 100× its starting length, at a stretch rate over 10%per second at a temperature of between 35° C. and 327° C. An expandedporous polytetrafluoroethylene (ePTFE) matrix in continuous film form inwhich is distributed a conductive particulate filler is produced.

The ePTFE matrix in continuous film form may be split to a desired widthby a means for slitting films to form a continuous slit film fiberhaving a substantially rectangular profile. The continuous slit filmfiber is subsequently stretched uniaxially in a longitudinal directionup to fifty (50) times its length by the method taught in Gore,previously reference herein. The second stretching step increases thestrength of the resultant fiber producing an expanded continuous slitfilm fiber. The increase in strength of the expanded continuous slitfilm fiber is a result of increased orientation of the ePTFE matrix. Forany specific conductive particulate filler, the amount of stretching towhich the continuous slit film fiber may be subjected is dependent onthe percentage of particulate filler present in the fiber. The greaterthe percentage of particulate filler, the less the continuous slit filmfiber may be stretched.

The expanded continuous slit film fiber may subsequently be subjected toa temperature in excess of 342° C. in order to perform an amorphouslocking step as taught in Gore specifically on column 3, lines 49-65. Iffully restrained longitudinally, the amorphous locking step furtherincreases the strength and density of the expanded continuous slit filmfiber.

Alternatively, prior to slitting, the ePTFE matrix in continuous filmform may be compressed and densified by a means for compressing, such asa pair of adjacent nip rollers, to reduce the thickness of the ePTFEmatrix in continuous film form, as taught in U.S. Pat. No. 4,985,296 toMortimer, Jr. incorporated herein by reference. Compression anddensification increases contact between individual conductiveparticulate filler particles thereby increasing conductivity of theePTFE matrix in continuous film form producing a thin ePTFE matrix incontinuous film form. To increase the strength of the thin ePTFE matrixin continuous film form, multiple layers of the coherent extrudate arestacked longitudinally and calendered upon one another forming a layeredarticle. The layered article is subsequently dried, expanded anddensified to produce a thin ePTFE matrix of greater strength whencompared to an analogous thin ePTFE matrix produced from a single layerof ePTFE matrix.

The thin ePTFE matrix may be subjected to the amorphous locking steppreviously described. The thin ePTFE matrix in continuous film form maybe slit to a desired width by a means for slitting films to form a thincontinuous fiber having a substantially rectangular profile.

The electrically conductive ePTFE fiber exhibit relatively high bulktensile strengths with relatively low volume resistivities. Conductiveparticulate filler distributed in the ePTFE matrix, while responsiblefor the fiber's volume resistivity, does not contribute to the fiber'sstrength. Rather, strength of the fiber is as a result of the amount ofPTFE present and the strength of that PTFE. However, the formation of anePTFE matrix, while increasing the strength of the matrix, also reducesits density and, therefore, increases its volume resistivity.

Expansion of the ePTFE matrix for increased bulk tensile strength andsubsequent densification of the ePTFE matrix for decreased volumeresistivity permits one to tailor the properties of the electricallyconductive ePTFE fiber.

It is possible to increase the conductivity of the fiber by increasingthe density of the fiber. The density of the fiber may be increasedthrough compression. Compression of the fiber may be accomplished bypassing the fiber through a means for compressing such as, for example,a pair of nipped rollers.

The density of the fiber may also be increased by subjecting the fiberto the previously described amorphous locking step which causes a degreeof shrinkage in the fiber. Densification of the fiber through theamorphous locking step is preferable when the profile of the continuousfiber is to be maintained rather than altered through a compressionstep.

Electrically conductive ePTFE fibers may have a range of volumeresistivities. A fiber with a volume resistivity of 10¹² ohms cm orless, as determined by the method described in ASTM D257-90, is definedherein as "electrically conductive" and has utility in providingarticles of manufacture with ESD control capabilities. The lower valueof volume resistivity is not critical and is limited by the conductiveparticulate used.

The electrically conductive ePTFE fiber may be subsequently formed intoa tow. The tow is formed by hackling the continuous monofilament fiberforming a fibrous tow web. This fibrous tow web is subsequently choppedinto short lengths thereby producing a staple comprising a matrix ofePTFE in which a conductive particulate filler is distributed. Choppingthe staple into shorter lengths produces a flock.

The electrically conductive PTFE fiber may be formed into the staticdissipative nonwoven textile material or may be intermixed with anamount of a synthetic fiber to form the inventive textile material. Theterm "synthetic" is defined herein to mean an article of non-naturalorigin. The synthetic fiber may be in any form of fiber. The syntheticfiber may be either inorganic or organic in nature.

Inorganic synthetic fibers may be selected from the group consisting ofglass and ceramic. Organic synthetic fibers may be selected from thegroup consisting of polyester, polyamide, polyphenylene sulfide,polypropylene, polyethylene, polyether ether ketone, polyimide,polyacrylonitrile, polyketone, copolyimide and polyaramid. The organicsynthetic fiber may be a fluorocarbon. Fluorocarbons of use in theinstant invention may be selected from the group consisting ofcopolymers of tetrafluoroethylene and perfluoropropyl vinyl ether,fluorinated ethylene propylene copolymer, ethylenechlorotrifluoroethylene, ethylene trifluoroethylene, polyvinyldifluoride, polytetrafluoroethylene (PTFE). Preferably the fluorocarbonis expanded porous polytetrafluoroethylene (ePTFE) described in U.S.Pat. No. 4,187,390 to Gore, incorporated herein by reference.

The intermixing of the electrically conductive PTFE fiber and thesynthetic fiber may be accomplished through mechanical interlocking,fusing, bonding, or felting process thereby forming the inventivetextile material in nonwoven form. Preferably, the inventive textilematerial is in a felt form.

The production of felt from PTFE fibers is generally described in U.S.Pat. No. 2,893,105 to Lauterback and is accomplished through a needlepunching process, incorporated herein by reference.

The synthetic fiber and the electrically conductive PTFE fiber arepreferably intermixed to form the inventive textile material at apercentage of the electrically conductive PTFE fiber to the syntheticfiber of between 1% and 10% by weight, more preferably between 2% and 5%by weight.

Referring now to FIG. 1, the inventive textile material 11 is in theform of a filter bag 10. The filter bag 10 has an outside surface 12 andan inside surface 13 and is useful for filtering material from fluidstreams. The inventive textile material may also be in the form of acloth.

The inventive textile material may be combined with a porous polymericmembrane to produce a nonwoven textile laminate. The nonwoven textilelaminate is static dissipative when tested by the test method disclosedherein. The porous polymeric membrane may be selected from the groupconsisting of nitrocellulose, triacetyl cellulose, polyamide,polyurethane, polycarbonate, polyethylene, polypropylene,polytetrafluoroethylene, polysulfone, polyvinyl chloride, polyvinylidinefluoride, acrylate copolymer and methacrylate copolymer. The porouspolymeric membrane is preferably expanded porous polytetrafluoroethylenemembranes. The porous polytetrafluoroethylene membrane used is preparedexpanding polytetrafluoroethylene as described in U.S. Pat. No.3,953,566, to obtain expanded, porous polytetrafluoroethylene. By"porous" is meant that the polymeric membrane has an air permeability ofat least 0.0002 m/min @20 mm water gauge. Air permeabilities of 146.3m/min @20 mm water gauge or more can be used. The membrane maypreferably have an air permeability of at least 7.8 m/min @20 mm watergauge, for use in gas stream filtration. Lower air permeabilitymembranes are useful in liquid stream applications.

The production of felt from ePTFE fibers as well as the formation offabric laminates from felt is taught in U.S. Pat. No. 4,983,434 toSassa, incorporated herein by reference.

Referring now to FIG. 2, the nonwoven textile laminate 21 is in the formof a filter bag 20. The filter bag 20 has an outside 22 and an inside23. The static dissipative nonwoven textile material 11 is depicted onthe inside 23 and the porous polymeric membrane 24 is depicted on theoutside 22. Arrangement of the static dissipative nonwoven textilematerial 11 and the porous polymeric membrane 24 may be reversed ifrequired. The filter bag 20 is useful for filtering material from fluidstreams. The nonwoven textile laminate 21 may also be in the form of acloth.

TEST METHODS Static Decay Test

To determine if a sample is "static dissipative", its static decayproperty is determined. The test used is the National Fire ProtectionAgency (NFPA) Code 99 Chapter 3 "Static Decay Test". Samples areconditioned at 21° C., 40% relative humidity. The static decay time ofthe samples are measured by charging the sample to 5000 volts and thentiming its discharge to 500 volts. A material that has a static decaytime below 0.5 seconds is considered static dissipative.

Volume Resistivity

The volume resistivity of the fibers are determined using the methoddescribed in ASTM D257-90, "Standard Test Methods for D-C Resistance orconductance of Insulating Material".

The following example is presented to further explain the teachings ofthe instant invention and not to limit the scope of the invention.Various modifications and equivalents will readily suggest themselves tothose of ordinary skill in the art without departing from the spirit andscope of the instant invention.

EXAMPLE

A felt of the present invention was produced in the following manner.

An electrically conductive ePTFE fiber was produced from a conductivetape of ePTFE produced from a dry mixture of 20% by weight conductivecarbon black (Vulcan XC-72R available from Cabot Corporation, BostonMa.) and 80% by weight fine powder PTFE resin. The tape was slit alongits length into two parts, expanded, and processed over a rotatingpinwheel to form a tow yarn. The tow yarn was then chopped to 11 cmlength staple fiber.

A similar staple was also produced from a synthetic fiber of expandedporous PTFE.

The electrically conductive ePTFE fiber and the synthetic fiber ofexpanded porous PTFE were opened on conventional carding equipment andthen collected in boxes for additional processing.

A blend of the two stable fibers was produced by hand mixing 5% byweight electrically conductive ePTFE fiber and 95% by weight syntheticfiber.

A woven scrim was used made of 440 decitex ePTFE weaving fiber, (Rastex®fiber, available from W. L. Gore and Associates, Inc., Elkton, Md.). Thescrim was constructed with a thread count of 16×8 threads/cm resultingin a weight of approximately 130 g/m².

The blended staple fibers was fed into conventional carding equipment.The carded web was crosslapped onto the scrim and tacked together by aneedle loom. A web was then crosslapped onto the other side of the scrimand needled again. The felt was needle punched several times tointerlock the staple fibers to the scrim sufficiently. This product wasthen heat set while being restrained in the cross machine direction forseveral minutes to improve it's thermal stability. The final felt had aweight of 600 g/m², an air permeability of 17 m/min @20 mm water gauge,and a thickness of 0.80 mm.

The needled felt was coated with an fluorinated ethylene propylenecopolymer (FEP) aqueous dispersion (T-120 available from E. I. duPont deNemours and Co.). The felt was then dried in loop dryer oven at 200° C.with a dwell time of 8 minutes. The dried FEP aqueous dispersion add onwas 3.5% by weight.

A layer of porous expanded PTFE membrane with an air permeability of 8.8m/min @20 mm water gauge was laminated to the coated side of the felt.The felt was subjected to sufficient heat, pressure, and dwell time tosoften the dried FEP aqueous dispersion. The resulting fabric laminatehad good bond strength between the porous expanded PTFE membrane and thefelt, and an air permeability of 2.8 m/min @20 mm water gauge withexcellent filtration efficiency of solid particulates.

The inventive nonwoven fabric laminate was tested to determine itsstatic decay properties. The fabric laminate was determined to be staticdissipative since it had a static decay time of less than or equal to0.01 seconds.

We claim:
 1. A static dissipative nonwoven textile material comprising:an electrically conductive polytetrafluoroethylene fiber intermixed withat least one other synthetic fiber, wherein the static dissipativenonwoven textile material has a static dissipative time of 0.5 secondsor less; and wherein the percentage of the electrically conductivepolytetrafluoroethylene fiber to synthetic fiber is between 1% and 10%by weight.
 2. A static dissipative nonwoven textile material asdescribed in claim 1 wherein the static dissipative textile material isin felt form.
 3. A static dissipative nonwoven textile material asdescribed in claim 1 wherein the synthetic fiber is an inorganicsynthetic fiber.
 4. A static dissipative nonwoven textile material asdescribed in claim 3 wherein the inorganic synthetic fiber is selectedfrom the group consisting of glass and ceramic.
 5. A static dissipativenonwoven textile material as described in claim 1 wherein the syntheticfiber is an organic synthetic fiber.
 6. A static dissipative textilematerial as described in claim 5 wherein the organic synthetic fiber isselected from the group consisting of polyester, polyamide,polyphenylene sulfide, polypropylene, polyethylene, polyether etherketone, polyimide, polyacrylonitrile, polyketone, copolyimide andpolyaramid.
 7. A static dissipative textile material as described inclaim 1 wherein the electrically conductive polytetrafluoroethylenefiber is conductive expanded porous polytetrafluoroethylene.
 8. A staticdissipative nonwoven textile material as described in claim 1 whereinthe percentage of the electrically conductive polytetrafluoroethylenefiber to synthetic fiber is between 2% and 5% by weight.
 9. A staticdissipative nonwoven textile material as described in claim 1 in theform of a filter bag.
 10. A static dissipative nonwoven textile materialas described in claim 1 in the form of a filter cloth.